The present application will pursue the following objectives: (1) Prolongation of allograft survival after in vitro culture: (a) determine optimal (least toxic to other cells) tissue culture conditions that will kill donor leukocytes (macrophages): (b) determine if these conditions will allow survival of thyroid and pancreatic tissues in culture as evidenced by histological examination and function: (c) determine if culture under these conditions will abrogate allograft response. (2) The role of macrophages in Immunosuppression induced by tumors: (a) test the hypothesis that tumors shift the population of macrophages toward a phenotype incapable of presenting antigen and (b) determine the mechanism by which tumors induce the synthesis of prostaglandins and arginase in macrophages.